A can of worms: estimating the global number of earthworm species

Abstract

Estimating the overall number of species for a given taxon is a central issue in ecology and conservation biology. It should be particularly topical in the case of soil organisms, which represent the majority of known species, but it still suffers from a considerable taxonomic knowledge deficit. We propose here an estimation of the global number of earthworm species based on the Joppa approach, which models taxonomic effort over time in order to estimate the total number of known and, as yet, unknown species in a given taxa. We also conducted regional assessments of species numbers, which allowed us to refine global estimates by providing an accurate overview of the taxonomic deficit in different latitudinal locations on the planet. Our results allow us to propose a conservative estimation of global diversity of the order of 30,000 species. This means that the 5,679 species and subspecies already described represent at best around 20% of the global number of earthworm species. Considering the current rate of new species description, we calculate that it would take at least 120 years to describe all the species existing on Earth. We discuss the strategies that could be developed to reduce the magnitude of this taxonomic deficit.